User Documentation

Assigning Devices to VMs

Sometimes you may need to assign an entire PCI or PCI Express device directly to a qube.
This is also known as PCI pass-through.
The Qubes installer does this by default for sys-net (assigning all network class controllers), as well as sys-usb (assigning all USB controllers) if you chose to create the USB qube during install.
While this covers most use cases, there are some occasions when you may want to manually assign one NIC to sys-net and another to a custom NetVM, or have some other type of PCI controller you want to manually assign.

Note that one can only assign full PCI or PCI Express devices by default.
This limit is imposed by the PC and VT-d architectures.
This means if a PCI device has multiple functions, all instances of it need to be assigned to the same qube unless you have disabled the strict requirement for FLR with the no-strict-reset (R4.0) or pci_strictreset (R3.2) option.
In the steps below, you can tell if this is needed if you see the BDF for the same device listed multiple times with only the number after the “.” changing.

While PCI device can only be used by one powered on VM at a time, it is possible to assign the same device to more than one VM at a time.
This means that you can use the device in one VM, shut that VM down, start up a different VM (to which the same device is also assigned), then use the device in that VM.
This can be useful if, for example, you have only one USB controller, but you have multiple security domains which all require the use of different USB devices.

R4.0

In order to assign a whole PCI(e) device to a VM, one should use the qvm-pci tool.
First, list the available PCI devices:

qvm-pci

This will show you the backend:BDF address of each PCI device.
It will look something like dom0:00_1a.0.
Once you’ve found the address of the device you want to assign, then attach it like so:

qvm-pci attach --persistent <vmname> <backend>:<bdf>

For example, if 00_1a.0 is the BDF of the device you want to assign to the “personal” domain, you would do this:

qvm-pci attach --persistent personal dom0:00_1a.0

R3.2

In order to assign a whole PCI(e) device to a VM, one should use the qvm-pci tool.
First, list the available PCI devices:

lspci

This will show you the BDF address of each PCI device.
It will look something like 00:1a.0.
Once you’ve found the BDF address of the device you want to assign, then attach it like so:

qvm-pci -a <vmname> <bdf>

For example, if 00:1a.0 is the BDF of the device you want to assign to the “personal” domain, you would do this:

qvm-pci -a personal 00:1a.0

Using Qubes Manager

The above steps can also be done in Qubes Manager.
Simply go into the VM settings of your desired VM, then go to the “Devices” tab.
This will show you a list of available devices, which you can select to be assigned to that VM.

Finding the right USB controller

Some USB devices are not compatible with the USB pass-through method Qubes employs.
In situations like this, you can still often get the USB device to work by passing through the entire USB controller to a qube.
However, with this approach one cannot assign single USB devices, only the whole USB controller with whatever USB devices are connected to it.
More information on using and managing USB devices with qubes is available on the USB page.
If you want assign a certain USB device to a VM by attaching the whole USB controller, you need to figure out which PCI device is the right controller.
First, check to which USB bus the device is connected (note that these steps need to be run from a terminal inside dom0):

lsusb

For example, I want assign a broadband modem to the NetVM.
In the output of lsusb it can be listed as something like this.
(In this case, the device isn’t fully identified):

Bus 003 Device 003: ID 413c:818d Dell Computer Corp.

The device is connected to USB bus #3.
Then check which other devices are connected to the same bus, since all of them will be assigned to the same VM.
Now is the time to find the right USB controller:

readlink /sys/bus/usb/devices/usb3

This should output something like:

../../../devices/pci-0/pci0000:00/0000:00:1a.0/usb3

Now you see the BDF address in the path (right before final usb3).
Strip the leading 0000: and pass the rest to the qvm-pci tool to attach the controller with the version specific steps above.

Possible issues

DMA buffer size

VMs with assigned PCI devices in Qubes have allocated a small buffer for DMA operations (called swiotlb).
By default it is 2MB, but some devices need a larger buffer.
To change this allocation, edit VM’s kernel parameters (this is expressed in 512B chunks):

PCI passthrough issues

Sometimes the PCI arbitrator is too strict.
There is a way to enable permissive mode for it.
See also: this thread and the Xen wiki’s PCI passthrough page.

NOTE: By setting the permissive flag for the PCI device, you’re potentially weakening the device isolation, especially if your system is not equipped with a VT-d Interrupt Remapping unit.
See Software Attacks on Intel VT-d (page 7)
for more details.

At other times, you may instead need to disable the FLR requirement on a device.
This will also weaken device isolation; see the “I created a usbVM…” entry in the FAQ for more details.

R4.0

Permissive mode and strict reset are options set as part of PCI device attachment.
If you’ve already attached the PCI device to a VM, detach it first either with Qube Manager or qvm-pci, then list the available PCI devices:

qvm-pci

This will show you the backend:BDF address of each PCI device.
It will look something like dom0:00_1a.0.
Once you’ve found the address of the device you want to assign, then attach it like so:

Running qvm-pci again should then show your PCI device attached with both the permissive and no-strict-reset options set.

Note again that in most cases you should not need either of these options set.
Only set one or more of them as required to get your device to function, or replace the device with one that functions properly with Qubes.

Note again that in most cases you should not need either of these options set.
Only set one or more of them as required to get your device to function, or replace the device with one that functions properly with Qubes.

Bringing PCI device back to dom0

By default, when a device is detached from a VM (or when a VM with an attached PCI device is shut down), the device is not automatically attached back to dom0.
This is an intended feature.
A device which was previously assigned to a VM less trusted than dom0 (which, in Qubes, is all of them) could attack dom0 if it were automatically reassigned there.

In order to re-enable the device in dom0, either:

Reboot the physical machine.

or

Go to the sysfs (/sys/bus/pci), find the right device, detach it from the pciback driver, and attach it back to the original driver.
Replace <BDF> with your full device, for example 0000:00:1c.2: